This invention relates to bricks and particularly spinel-magnesia bricks available for use in rotary cement kilns.
What a burning zone is composed of is of great importance for the rotary cement kiln. It has been common up to now that basic bricks such as chrome-magnesia brick, and spinel (MgO.Al.sub.2 O.sub.3) are used to line such a burning zone. It has been, however, known that a stable coating of cement material should be applied to the work surface of the brick to prolong the life thereof.
It has been found that cement material and basic brick are required to suitably chemically react on each other to have the coating of cement material applied to basic brick, and that a stronger chemical reaction renders brick molten but no chemical reaction or weak chemical reaction involves a poor coating application. An iron modulus, that is, Al.sub.2 O.sub.3 /Fe.sub.2 O.sub.3, ingredient of cement material is less than 1.6, preferably, less than 1.5 to have the coating applied to brick. Otherwise, a silica modulus, SiO.sub.2 /(Al.sub.2 O.sub.3 +Fe.sub.2 O.sub.3) may be less than 2.5. At any rate, reduction in the iron modulus would be more effective.
The principal ingredients, main minerals of cement material are 2CaO.SiO.sub.2 ; 3CaO.SiO.sub.2 and 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3. Among others, 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3 is used for application of the coating. The melting points of 2CaO.SiO.sub.2 ; and 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3 are 2,130.degree. C., and 1,410.degree. C., respectively. 3CaO.SiO.sub.2 is resolved into CaO and 2CaO.SiO.sub.2 at a temperature of 1,900.degree. C. The latter provides a conversion from type .beta. to type .gamma. with increase in volume of about 10% at a temperature of 725.degree. C. in a cooling process so that the coating may readily break and exfoliate. On the other hand, 3CaO.SiO.sub.2 is resolved into 2CaO.SiO.sub.2 and CaO at a temperature of about 1,200.degree. C. but fails to obtain complete stability. 4CaO.Al.sub.2 O.sub.3.Fe.sub.2 O.sub.3 is contained to a small extent but its penetration to only the surface layer of basic brick not only carry out application of coating in an early stage of the rotary cement kiln but also effectively stabilize coating in operation. Thus, the lower the iron modulus is, the easier coating is applicable. In this connection, it is noted that too much reduction in the iron modulus of cement material should be avoided which might otherwise affect quality of cement.
The use of high purity magnesia clinker in the conventional spinel magnesia brick has been proposed to its texture thereby to minimize impurities such as Fe.sub.2 O.sub.3 and the like to the utmost extent. As a result, the amount of contained Fe.sub.2 O.sub.3 in brick is less than 0.1% to obtain brick of very high iron modulus as shown in Table 1. Supposing that 50% each of cement ingredient and brick are mixed with each other in the area of contact between cement material and brick, the iron modulus would be very high leading to difficulty in application of coating.
In the manufacture of the conventional spinel-magnesia brick, material of very high purity has been used due to synthesis of spinel. For instance, high purity seawater magnesia has been used as magnesia material and Fe.sub.2 O.sub.3 as impurity is maintained below 0.1%. Similarly, a high purity article which contains more than 99% of Al.sub.2 O.sub.3 has been used as alumina material and Fe.sub.2 O.sub.3 is also maintained below 0.1%. The conventional spinel-magnesia brick in which these high purity materials are used, entails a high cost of production and provide a poor coating when practised in the rotary cement kiln. This results in something undesirable from the view point of longevity. It has been found in the spinel-magnesia brick that what has caused the poor coating may be the result of formation of .gamma.-Al.sub.2 O.sub.3 in the area of spinel and cement clinker.